This invention pertains to rolling mills for the production of metal products. An exemplary rolling mill would be a hot strip mill in which steel slabs are converted to sheet and strip products. This invention also relates to other rolling mills in which the products are, for example, plates, bars, structural shapes and rails and in which case the starting shapes are referred to as slabs, blooms, billets, etc. Usually, the output of the hot strip mill is further processed by cold rolling. The hot strip mill is preceded by a reheat furnace for raising the temperature of the slabs prior to entry into the hot strip mill. The hot strip mill is comprised of a roughing mill and a finishing mill. Control of the temperature of the slab throughout the hot rolling mill is essential. Three temperatures are considered of particular importance; the temperature of the slab as it exits the furnace, the temperature of the slab exiting from the roughing mill and the finishing temperature at the end of the finishing mill. Throughout the hot rolling process, there is a large drop in temperature. While the specific temperatures depend upon the grade and size of the product, hot rolling usually begins near 2200.degree. F. (1200.degree. C.) and finishes well above 1300.degree. F. (700.degree. C.). The function of the reheat furnace is to bring the slabs to the correct temperature to begin the hot rolling process.
Control of the reheat furnace to deliver the slabs at the desired starting temperature (herein the steel "discharge temperature") is no simple matter. It is very difficult to accurately measure the temperature of the slabs in the reheat furnace or even immediately after extraction. The slabs are covered with scale and do not have a uniform temperature through their thickness. Moreover, few mills have the luxury of continuously processing slabs of identical size and grade. These are constantly changing making control of the extract temperature more difficult. Other factors are also variable such as the rate at which the slabs are moved through the reheat furnace and the temperature of the slabs entering the reheat furnace.
Computer control has been implemented in mills for controlling the reheat furnaces, the roughing mill, and the finishing mill. Often each area has its own computer control coordinated by a central computer so that the product has the desired thickness, width and temperature. The desired or aim furnace discharge temperature (herein the "aim discharge temperature") is established so that under the expected operating conditions of the roughing mill, the slab will have a desired temperature on leaving the roughing mill (herein the "rougher exit temperature"). This invention relates to methods of improving the reliability of the discharge temperature. In other words, it relates to insuring that the actual discharge temperature (which cannot be directly measured) will result in the aim rougher exit temperature.
Mill practice tables exist for the roughing and finishing mills that establish for a given grade and size of slab and the desired finishing temperature, the aim discharge temperature and the aim rougher exit temperature. These tables are stored in the central computer, for example. Hence, as soon as a slab enters the reheat furnace on it way to the rolling mill, these aim temperatures are established by reference to the mill practice table.
To bring the slab to the aim discharge temperature, various heating zones of the reheat furnace are individually controlled. The control of the various zones to bring about the aim discharge temperature is based upon a reheat furnace model that is theoretically and empirically developed. The model is used two ways. It is used to established the desired time and temperature in each zone of the reheat furnace as the slab passes therethrough to bring about the aim discharge temperature. Because the control of the heating zones of the reheat furnace is not perfect, the model can be used to calculate or predict the discharge temperature based upon the measured conditions in each zone as the slab passes therethrough. It is practically impossible to perfectly predict the slab temperature within the furnace since conditions within and around the furnace are constantly changing. Seasons change, linings wear, gas pressures vary, thermocouples drift, etc. Moreover, the size and composition of the product may vary. These uncontrolled and unaccounted for conditions may be short-term or long-term changes that make the reheat furnace model less accurate than desired for predicting the actual discharge temperature.
Also, the conditions in the rolling mill itself may vary day to day or from operator to operator. Water sprays may be turned on or off, certain roughing mill stands may be bypassed for repair, etc. This can change what is required as an aim discharge temperature in order to achieve the aim rougher exit temperature.
The first temperature in the hot mill that can be accurately measured is the rougher exit temperature. An optical pyrometer is the usual device to measure this temperature. It has already been proposed to use a measure of this temperature to tune the operation of the reheat furnace. See "Automatic slab heating control at Inland's 80-in. hot strip mill," Veslocki et al., AISE Year Book, 1986. The procedure described therein requires the use of a roughing mill model to predict the rougher exit temperature of a slab given the discharge temperature provided by the reheat furnace model and times in the various mill stands and other measurable parameters of the roughing mill. The difference between the measured rougher exit temperature and that predicted by the roughing mill model is used as feedback for correcting the discharge temperature predicted by the reheat furnace model. This method, however, does not attempt to control an aim rougher exit temperature. It only attempts to "improve" the calculated furnace discharge temperature based upon measured rougher exit temperature and the roughing mill model. Unfortunately, roughing mill models, like all models, are subject to short and long term drift due to unmeasured parameters. As with reheat furnace models, there are many different roughing train models in use, each with its own drawbacks.